Photoelectric register control of presses for printing,folding or cutting webs



April 15, 1969 E HSTLEY TAL 3439,176

9 PHOI'OELECTRIC REGISTER CONTROL 0F PRESSES FOR PRINTING, FOLDING ,vu/.sf Genen/eran 0R CUTTING WEBS Filed Feb. 2l. 1966 (ltorneys United States Patent O Int. Cl. G01n 21/30 U.S. Cl. 250--219 11 Claims ABSTRACT OF THE DISCLOSURE In the register control of preprinted webs, to permit a single transverse register mark located between adjacent print areas to be distinguished from marks within the print areas, the unit for sensing the mark consists of a number of light-sensitive elements -arranged so that they view transversely spaced points in the plane of the web. A logic circuit ensures that a register pulse is generated only when those elements in the path of the mark indicate a clear space, a mark and a clear space, in that order and during the same period elements outside the path of the mark continuously indicate a clear space.

When a continuous web of paper is printed with a succession of similar images and other operations are to be performed on the web in positions controlled by the positions of the first images, some form of register control becomes necessary. Examples of such systems are multi-colour presses in which the subsequent colour component images which are printed on the web must be maintained in register with the first colour component image, and folding machines in which the operation of the folding machine must be synchronised with the arrival of the preprinted images. Register control is normally carried out by printing register marks on the web and using light-sensitive cells to detect the passage of these marks.

One of the most awkward practical problems of a photoelectric register control is to arrange that the error- ,measuring circuits are controlled by the register marks only and not `'by the remainder of the print. The simplest solution is to have a clear strip of paper running `along the edge of the web .and to place the register marks in this. Frequently, it is impractical to have this arrangement, however, because paper is expensive and printers are not prepared to provide margins which are used only for the register marks.

In an alternative arrangement the register marks ,are printed in an area between the picture areas on the web and a small impulse generator is coupled to the printing press and is used to switch on and off the photocell cir cuits at moments such that they are only effective when they are scanning the transverse strip of the web be tween the picture areas. This prevents the photocells from being influenced by marks within the picture areas themselves, but requires that, in order to accommodate difierent Webs, it is necessary for the printer to align the impulse generator with respect to the photocell and register marks at the beginning of each printing run.

In ,most cases it is necessary to run the press and obtain approximate colour register manually, stop the press and synchronise the gate generators and then restart the press. Since there is a gate generator for each pair of colours, this operation consumes time and produces waste material due to the run up and `down of the press. Additionally, there is the possibility on gravure presses of the 3,439,176 Patented Apr. 15, 1969 web sticking to the printing cylinder, thus causing web break and further down time and waste material.

The present invention has for its object to make this pre-alignment unnecessary and thus to save press time and material.

One `approach to this problem which has been proposed is to print a number of register marks as a code and then to use logic circuits to determine when this code of marks has passed the sensing head. However, this involves a larger number of marks on the web in order to form a recognisable code and it is frequently diiicult to find sucient clear space on the web to accommodate these marks.

According to the present invention, a transverse register mark of standard form is sensed by a detector head including light-sensitive elements so arranged that they view points in the plane of the web which are transversely spaced from one another, some of the light-sensitive elements viewing points which are in the path of different portions of the register mark `and at least one other light-sensitive element viewing a. point which is transversely displaced with respect to the path of the register mark; a discriminator circuit receives signals from the light-sensitive elements and provides a register pulse only when the levels of the output signals of the lightsensitive elements are such as to indicate in succession a clear space sensed by all the elements, a. mark sensed by the elements which are in the path of the register mark and clean space sensed Iby the other element or elements, and a clear space against sensed by all the elements. In the preferred form of the invention,I the output sig nals from the inner elements representing the sensing of a mark, are additionally applied to a circuit controlled by a signal representing the web speed, and the output pulse is generated only if an output signal from this circuit, representing the dimension of the mark in the direction of travel of the web, is less than a predetermined amount. The output pulse may be used to switch on a gate for a scanning head of conventional form. The total scanning head may thus consist of the photocell array described above followed by two photocells in the path of register marks to be compared, one of which may be that already viewed by the photocell array.

The light-sensitive elements may be individual small photoelectric cells, each with its own output to a logic circuit, or they .may be embodied in a scanistor, for example of the form described in the proceedings of the Institute of Electrical and Electronic Engineers, vol. 52, December 1964, pages 1513-1526. Such a scanistor includes a linear array of photo-diodes the outputs of which can be sampled in the order in which they are arranged in the line by the application of a sawtooth waveform across the terminals of the device. The scanistor described in the .above article is in the form of a block of silicon with diodes and connections diffused and etched on to it, but if desired a scanistor including discrete components can be used in its place.

It will be seen that the invention permits the recognition of the register mark by an arrangement of light-sensitive cells which, with their associated discriminator crcuits, reject marks of incorrect length and that the discriminator circuits are such that they also reject marks which do not have ahead of and behind them a clear space. Moreover, by measuring the duration of the signals representing the clear spaces and the mark, and lmodifying them when necessary in accordance with the press speed, marks can also be rejected if the length of the clear space ahead and behind the mark in the direction of travel of the web are less than a predetermined minimum and if the thickness of the mark in this direction is greater than a predetermined minimum.

In order that the invention may be better understood,

one example will now be described with reference to the accompanying drawing, which shows diagrammatically the arrangement of the photocells and web, together with the discriminator circuit for interpreting the signals derived from the photocells. The dimensions of the mark and clear spaces have been exaggerated for the sake of clarity.

In the drawing, the web 1, which is travelling in the direction of the arrow, carries a register mark 2 between two printed areas 3. A light source 4 and cylindrical lens 5 projects a narrow line of light 6 onto the web 1. It will be seen that as the web moves forward the register mark 2 will pass through the line of light.

Five photo-diodes 7, 8, 9, and 11 are arranged so that they view small circular areas 7a, 8a, 9a, 10a, and 11a spaced along the line of light. While the printed area 3 is passing under the line of light, the signals from the photo-diodes will vary in level in accordance with the density of the printed matter. As the trailing edge of the printed area leaves the line of light, the output signals from the tive photo-diodes will go to their white level and will remain there until the register mark 2 enters the beam of light. At this point, the signals from the photodiodes 8, 9 and 10 will go to their black level. When the register mark passes beyond the line of light these three signals will be restored to their white level, and thereafter all signals will remain at their white level until the next printed area 3 enters the line of light.

The output signals from the photo-diodes 7 to 11 are passed through A.G.C. amplifiers 12, by means of which the difference between white and black levels is automatically adjusted to a predetermined value, to restorer circuits 13 by means of which the white levels of all signals are brought to the same level.

The outputs of the tive restorer circuits are applied to a Schmitt trigger circuit S1 which has a zero output when all photo-diodes see clear paper and switches over to produce an output when any diode sees a darkened area. When the output of the Schmitt trigger circuit becomes zero, a Miller integrator M1 begins to charge. The rate of charge is controlled by the signal on the conductor 14a, which is derived from a tacho-generator and is proportional to the web speed. In this example, the Miller integrator charges at the rate of l0 volts for 1,/4" of web travel. If at any time during the charging of the Miller integrator a mark appears in the iield of the photodiodes, the Schmitt trigger circuit will provide an output and the Miller integrator will rapidly discharge. If the Miller integrator is permitted to charge to its full extent (i.e. for 1A of web travel), it will switch on a trigger circuit T1, indicating that all diodes have seen clear space for 1A web travel. The trigger circuit T1 cannot be switched off by subsequent changes of condition of the Miller integrator M1. The outputs of the tive restorer circuits 13 are also applied to an AND circuit A1, the outputs corresponding to the photo-diodes 7 and 11 being first applied through inverter circuits. Consequently the AND circuit A1 provides an output only if the inner three diodes 8, 9 and 10 see a mark while the outer diodes 7 and 11 see clear space. If, after trigger circuit T1 has been switched on, the Schmitt trigger circuit S1 provides an output but the AND circuit A1 does not (indicating that the photo-diodes have detected a mark but that it is not of the form required by the circuit A1), the AND circuit A2 provides an output which switches oti trigger circuit T1 and thereby resets the system.

If, however, both S1 and A1 produce an output, indicating that the mark is of the required form, T1 remains on and the output of the AND circuit A1 is applied through the gate circuit G, which has been turned to its signal-passing condition by the trigger circuit T1, to a monostable trigger circuit T2. This circuit is controlled by a signal on conductor 14b from the the tacho-generator and is timed to revert to its original condition after 1A of web travel.

When the monostable trigger circuit T2 reverts to its original condition, the differentiating circuit D to which it is connected, applies pulses to the AND circuit A3 and to the Miller integrator M1. The function of the AND circuit A3 is to switch off trigger circuit T1 if there is an output from S1 when the differentiating circuit generates its output pulse. Thus, if the mark detected by the photodiodes is too thick (i.e. extends too far in the direction of web travel), trigger circuit T1 is reset.

If, however, the mark detected is a true register mark, circuit A3 will not operate. The signal applied to the Miller generator M2 from the differentiating circuit causes the Miller circuit to charge, again at a rate controlled by a signal on conductor 14C, derived from the tacho-generator. In this example, the Miller generator charges to the required level after the passage of l/s of web. However, the Miller generator will `only charge to this level if the trigger circuit T1 remains on for the whole of the time and if the Schmitt circuit S1 gives Zero output. If these conditions are maintained, at the end of its charging period, the Miller circuit operates an output pulse generator TG and resets the trigger circuit T1 by way of the conductor 15. In this example, the output pulse is not very precisely defined in relation to the position of the mark and therefore it is used to open a gate in a standard scanning head which includes photocells so placed that they view register marks (possibly including the mark 2 of the drawing) at a point further along in the direction of web travel than the photo-diodes 7-11. Pulses corresponding to these latter register marks are compared and in the usual way time differences are caused to initiate register corrections.

As stated earlier, the separate photocells with their separate output conductors can be replaced by a scanistor having a single output line. In such a case, the output signal represents the result of scanning transversely across the scanistor and therefore represents the density values obtained in a transverse scan across the corresponding part of the web. This transverse scan is repeated a number of times between the instants when the trailing edge of one picture area leaves the lield of the scanistor and the leading edge of the next picture area comes into the field of the scanistor. The condition for the generation of a register pulse may, for example, be that the irst ten transverse scans result in a signal of a constant value representing the white level, the next three transverse scans result in a signal which at the beginning of each line is at white level, followed by a jump t0 black level, followed by a reversion to white level; and the last ten transverse scans again result in a continuous white level signal. The logic circuits required to deal with this are designed along the same principles as the logic circuit described above, with modifications to deal with the fact that the register mark is now represented by three successive black level signals (in the example given above) each occupying ya part of the corresponding transverse line scan.

It is not essential to project a line of light on to the web. The web can be generally illuminated or a pool of light can be projected on to it, provided that the area viewed by each photocell is suitably restricted. If no tacho-generator is used the output of a photocell cannot be related to web travel but a measurement of web travel can still be made by projecting a number of parallel bars of light on to the web at predetermined spaeings.

In the form described, two photo-diodes are transversely displaced on opposite sides of the path of the register mark. The absence of a signal from these photodiodes when the signals from the remaining photodiodes indicate the presence of a mark on the web provides a measure of the length of the mark. lf however, the register mark extends inwards from one edge of the web, it is possible to use only one photo-diode transversely displaced with respect to the path of the register mark.

We claim:

1. A method of register control of webs, in which a transverse register mark of standard form is sensed by arranging light-sensitive elements so that they View points in the plane of the web which are transversely spaced from one another, some of the light-sensitive elements viewing points which are in the path of dicerent portions of the register mark and at least one other lightsensitive element viewing a point which is transversely displaced with respect to the path of the register mark, and generating a register pulse only when the levels of the output signals of the light-sensitive elements are such as to indicate in succession a clear space sensed by all the elements, a mark sensed by the elements in the path of the register mark and clear space sensed by the other element or elements, and a clear space sensed by al1 the elements.

2. A method in accordance with claim 1, in which signals from the inner light-sensitive elements representing the sensing of a transverse mark during the longitudinal passage of the web are modified by a signal representing the web speed, so that the resultant signal represents the dimension of the mark in the direction of travel of the web, and in which the register pulse is generated only if this dimension is less than a predetermined amount.

3. A method in accordance with claim 2, in which the output signals from the inner elements representing the clear spaces ahead of and behind the `mark are modied by a signal representing the web speed to provide a resultant representing the dimension of these clear spaces in the direction of travel of the web, and in which the register pulse is generated only if these dimensions are greater than predetermined amounts.

4. A method in accordance with claim 1, in which the said register pulse opens a gate in the circuit of a photoelectric detector head arranged to View a point in the path of the register mark, and in which a pulse derived from the said detector head while the gate is open is compared with a further register pulse to produce a difference signal for register correction.

5. Register control apparatus including, for sensing a single transverse register mark of standard form, a detector head including light-sensitive elements so arranged that they view points in the plane of the web which are transversely spaced from one another, some of the lightsensitive elements viewing points which are in the path of different portions of the register mark and at least one other light-sensitive element viewing a point which is transversely displaced with respect to the path of the register mark, a discriminator circuit receiving signals from the light-sensitive elements and including a first circuit responsive to white-level signals from all the lightsensitive elements, a second circuit conditioned by the iirst circuit and responsive to the combination of signals from the light-sensitive elements in the path of the register mark corresponding in level to the mark density and white-level signals from other light-sensitive element or elements, and a third circuit conditioned by the second circuit and responsive to white-level signals from all the light-sensitive elements, the apparatus further including means for generating a register pulse only in response to the successive operation of the iirst, second and third circuits.

6. Apparatus in accordance with claim 5, including a tacho-generator for generating electric signals representing the `web speed, a circuit responsive to the web speed signal and to a mark-representing signal from one of the lightsensitive elements to generate a signal representing the dimension of the mark in the direction of travel of the web, and means for inhibiting the register pulse generator if the said dimension-representing signal is more than a predetermined amount.

7. Apparatus in accordance with claim 5, including a tacho-generator for generating an electric signal representing the web speed, a circuit responsive to the web speed signal and to the signals from a light-sensitive element representing the clear space ahead of and behind the mark, to generate electric signals representing the dimensions of these clear spaces in the direction of travel of the web, and means preventing the generation of a register pulse unless the said dimensions represented by the signais are greater than predetermined Values.

8. Apparatus in accordance with claim 5, in which the second circuit includes an AND circuit connected to the light-sensitive elements and having inverter circuits in certain 'of its input connections, the AND circuit providing an output when the register mark is sensed, a gate circuit which passes a signal from the AND circuit when conditioned by the iirst circuit, and a timing circuit which is switched on by the signal passed by the gate circuit.

9. Apparatus in accordance with claim 8, in which the third circuit includes a further timing circuit which is switched on at the end of the period timed by the rst timing circuit, and means for stopping the further timing circuit if any of the light-sensitive elements senses a mark.

1?. Apparatus in accordance with claim 5 in which the array of light-sensitive elements includes three such elements arranged to view points in the path of the register mark and two further elements arranged to view points transversely displaced on opposite sides of the path of the register mark.

11. Apparatus in accordance with claim 5 in which the light-sensitive elements are arranged in a scanistor.

References Cited UNITED STATES PATENTS 7/ 1942 Sorkin. 5/1944 Sorkin.

U.S. C1. X.R. 226-27 

